The evolution of radio interface standards has been strongly focused on increased data rates, and in the Third Generation Partnership Project (3GPP) Release 7 a technology known as Multiple Input Multiple Output (MIMO) was introduced. Such technology uses multiple antennas at both the transmitter and receiver to theoretically double the downlink data rate using multiple data stream transmission.
Certain User Equipment (UE) categories are able to use MIMO technology, but other legacy UE categories are not be able to use MIMO technology. A network must be able to support both MIMO enabled and non-MIMO enabled UEs. Support for legacy UEs may be provided by transmitting all system vital information and traffic channels on a single antenna. However, if there are separate Power Amplifiers (PA) for the multiple antennas, which is typically the case, the utilization of the PAs is suboptimal. This is because one antenna may be transmitting and receiving much more data than another antenna which represents an under utilisation of resources. In effect there is no power sharing between the PAs.
The problem of uneven power sharing may be alleviated by using Butler matrices at a Base Transceiver Station (BTS), also known as the NodeB. Such Butler matrices distribute the load equally over the PAs, but have the drawback of requiring more hardware and introducing a power loss. Furthermore, Butler matrices are not always useable for power balancing if the data streams from the transmit antennas are correlated, which is the case for example with single stream MIMO.
Another solution is to transmit all channels not using MIMO from the antennas through the use of Space-Time Transmit Diversity (STTD) encoding, which is an open loop transmit diversity scheme standardized in 3GPP Release 99. Such STTD encoding is supported by most UEs on the market. However, even though STTD transmission alleviates the PA power balancing problem and may be beneficial for common channels, there is less benefit for dedicated channels and in particular the High-Speed Downlink Shared Channel (HS-DSCH). The use of STTD encoding may actually harm the performance in certain cases, especially on the HS-DSCH which is a shared channel and a scheduled resource.
Further problems are also encountered with dual-stream MIMO transmission when providing mobile communication systems that support both MIMO enabled and non-MIMO enabled UEs. Typically it is assumed that the two data streams transmitted from each MIMO antenna have the same power, particularly in the case of Channel Quality Indicator (CQI) reporting when each UE calculates the quality of each data stream. In practice this may not always happen due to restrictions at a scheduler device within the BTS which is a controller device that allocates radio resources and when data for a plurality of UEs is to be transmitted. In general, to support both MIMO enabled and non-MIMO enabled UEs a MIMO precoder may be used in series with a secondary precoder, with each precoder imparting respective matrices on the data stream. At a data stream level the power allocation input to the MIMO precoder may be uneven, and the two precoders then distribute this power over the two PAs. In some instances a power imbalance may then be seen between the PAs, which is undesirable because it is a suboptimal use of resources.
In another scenario, if Ptot is the total power available at two PAs, and Ptot/2 is allocated to one data stream at one PA, and if a second data stream is formed by the scheduler device when a transmit buffer of the BTS only contains a small amount of data, the transmit power of the second data stream at the second PA may be less than Ptot/2. Such a scenario also provides an imbalance of power between the data streams from the PAs which is undesirable.
Other situations may also lead to the PAs allocating different power to the two data streams for dual stream MIMO operation. In one scenario this may be caused by power calculation rounding errors when calculating the number of bits for an available transport format. In another scenario this may be caused when one of the MIMO data streams is required to be retransmitted if Hybrid Automatic Repeat Request (HARQ) is used. When using HARQ, if a radio channel has deteriorated since an original transmission, one data stream may need to be transmitted using more power than used for the initial transmission. If Ptot/2 was used for the initial transmission of one data stream this will mean that the retransmitted stream will have a power of more than Ptot/2, and the other stream will be transmitted with less than Ptot/2, which is undesirable for the above reasons.